The present disclosure is related generally to the field of semiconductor light emitting devices, and more specifically to an architecture for an improved high-Al content, low defect quantum well light emitting device formed directly on a final substrate.
In the III-V compound semiconductor family, the nitrides have been used to fabricate visible wavelength light emitting device active regions. They also exhibit a sufficiently high bandgap to produce devices capable of emitting light in the ultraviolet, for example at wavelengths between 290 and 400 nanometers. In particular, InAlGaN systems have been developed and implemented in visible and UV spectrum light emitting diodes (LEDs), such as disclosed in U.S. Pat. No. 6,875,627 to Bour et al., which is incorporated herein by reference. These devices are typically formed on an Al2O3 (sapphire) substrate, and comprise thereover a GaN:Si or AlGaN template layer, an AlGaN:Si/GaN superlattice structure for reducing optical leakage, an n-type electrode contact layer, a GaN n-type waveguide, an InGaN quantum well heterostructure active region, and a GaN p-type waveguide region. In addition, the complete device may also have deposited thereover a p-type AlGaN:Mg cladding layer and a capping layer below a p-type electrode.
While significant improvements have been made in device reliability, optical power output, and mode stability, the performance of the nitride-based lasers and light emitting diodes emitting in the ultraviolet (UV) is still inferior to that of their blue or violet counterparts. It is particularly true that for deep UV lasers and light emitting diodes operating at wavelengths below 340 nm, the nature of the substrate and template layer have a critical impact on the overall device performance. For example, differences in lattice constant between the substrate and the structural layers of the device significantly affects optical output and device lifetime. While Al203 (sapphire) as a substrate has numerous advantages, it is highly lattice mismatched to the structural layers of typical deep UV epi-layers. The prior art AlGaN template layer formed over the typical Al203 substrate mitigates the problem somewhat, but the resulting crystal quality of the high aluminum-containing structural layers in typical deep UV light-emitting devices utilizing these templates are still very poor.
The dislocation densities in AlGaN or AlN template layers on sapphire are typically in the mid 109 to high 1010 cm−2 range. As a consequence, the external quantum efficiencies of deep UV light emitting diodes in the 290 nm to 340 nm range are still well below the external quantum efficiencies for blue GaN-based LED structures. The high dislocation densities also reduce the operating lifetime of devices utilizing such template layers.
Efforts to improve the quality of the LED structure in the ultraviolet range on AlxGa1-xN/sapphire templates have presented significant challenges due to the high defect density of epitaxial layers formed over the AlGaN crystallographic template. These defects tend to propagate upward, perpendicular to the layer planes, in the direction of crystal growth, forming features known as threading dislocation defects (or simply threading dislocations). If not mitigated, threading dislocations can permeate throughout the structure and reach the active layer, where the transmitted defects compromise light emission efficiency through non-radiative recombination.
In many cases, mechanical stresses lead to cracks in the heterostructure formed thereon. These issues are exacerbated when the Al content of layers formed above the AlGaN/sapphire system increases. Yet, as previously mentioned, an increased Al content (e.g., up to ˜50% in the MQWH active region of a 280 nm light emitting diode, and 60% to 70% in the surrounding AlGaN current and optical confinement layers) is required to obtain devices which emit in the mid- to deep-UV.
Various groups have published approaches to dealing with these shortcomings. All references referred to herein, and specifically each of the following references, are incorporated herein by reference. For example, Han et al., Appl. Phys. Lett, Vol 78, 67 (2001), discuss the use of a single AlN interlayer formed at low temperatures to avoid strain development. This low-temperature AlN interlayer approach has proven unsuccessful in the case of heterostructure growth with high Al mole fractions. Nakamura et al., J. J. Appl. Phys., vol. 36, 1568 (1997) has suggested short period GaN/AlGaN superlattice layers as a way of extending the critical layer thickness of AlGaN films grown pseudomorphically on GaN/sapphire. But the average Al mole fraction in these AlGaN/GaN systems is at such a low level (˜10% or less) that it is not compatible with deep UV light emitting diodes. Chen et al., Appl. Phys. Lett., vol. 81, 4961 (2002) suggests an AlGaN/AlN layer as a dislocation filter for an AlGaN film on a AlGaN/sapphire template. But again, the AlGaN/sapphire template presents the aforementioned series resistance problem. And Wong et al. in U.S. patent application Ser. No. 11/356,769, filed on Feb. 17, 2006, proposes a GaN/AlN superlattice formed between the GaN template layer and the MQWH active region. But again, the GaN template layer must be removed prior to light output for such a device.
There is a need for a UV light emitting device with improved operation characteristics. Accordingly, there is a need for a method and structure facilitating a high Al content MQWH active region with reduced threading dislocations, cracking, and related damage.